Efficiency of monolayers in evaporation suppression from water surface considering meteorological parameters

The evaporation from water reservoirs has become a global issue due to climate change, limited water resources, and population growth. In this research, three emulsions of octadecanol/Brij-35 (4:1), hexadecanol/Brij-35 (4:1), and a combination of the alcohols with Brij-35, octadecanol/hexadecanol/Brij-35 (2:2:1), were used in water. One-way ANOVA was applied to compare the mean of evaporation in different chemical and physical methods, and factorial ANOVA was used to investigate the main and interactional effects of different meteorological parameters on the rate of evaporation. Results showed that two physical methods of the canopy and shade balls performed better than the chemical methods, with reductions of 60 and 56% in evaporation, respectively. Among the chemical methods, the octadecanol/Brij-35 emulsion had a better performance with 36% of reduction in evaporation. One-way ANOVA results showed that among the chemical methods, only the octadecanol/Brij-35 had no significant difference with shade balls with a 99% probability level (P < 0.01). On the other hand, factorial ANOVA showed that the temperature and relative humidity had the highest effect on evaporation. Octadecanol/Brij-35 monolayer had a lower performance than two physical methods at low temperatures, but after increasing the temperature, its performance improved. This monolayer had a good performance at low wind speed compared to physical methods; however, with increasing wind speed, its performance was severely affected. For temperatures of over 37 °C, the evaporation rate increased more than 50% when the wind speed had changed from 3.5 m/s to more than 8.7 m/s.


Introduction
The world is under pressure to make optimal use of water resources because of increasing environmental concerns; for example, in Australia, the rate of evaporation is 2 m per year, and the average rainfall is 500 mm per year (Craig 2005). At such heat, about 95% of the rainfall evaporates before it falls to the ground. Studies have shown that more than half of the water accumulated behind dams and small reservoirs is lost due to evaporation (Craig 2005). Reservoir water loss in hot and dry areas can be significant enough to offset the positive effects of water storage behind dams (Zhang et al. 2017). Field measurements show that the rate of evaporation from Texas reservoirs in the USA is about 7.53 billion cubic meters per year, equivalent to 61% of agricultural need or 126% of the total drinking water need of the state in 2010. Evaporation rate from reservoirs will intensify during periods of recent drought (Wurbs and Ayala 2014). Another study on Texas reservoirs shows that about 20% of the useful volume of reservoirs is wasted annually due to evaporation. According to this study, in five coastal basins, reservoir evaporation losses were higher than the minimum freshwater flow required to maintain sustainable ecosystem health (Zhang et al. 2017). A study on 721 dam reservoirs in the USA showed that the amount of water lost from these reservoirs was more than 30 billion cubic meters per year, equivalent to 93% of the annual public water supply in the USA (Gao et al. 2008). The increase in evaporation rate from dam reservoirs due to climate change and the increase in reservoir water level due to sediment deposition Responsible Editor: Guilherme L. Dotto were other worrying points in this regard (Gao et al. 2008). Due to evaporation, 17.88 million cubic meters of water was lost from Qaraoun Dam in Lebanon during 2013. If this amount of water was used for the electricity generation, it would be worth $850,000, which only contained 5.1% of the loss. On the other side, the impact of evaporation on irrigation has been more significant, and the damages of evaporation in this sector are estimated about $43 million (Bou-Fakhreddine et al. 2019).
Today, various tools have been used to deal with surface evaporation, such as shade cover structures, covering the water's surface with thin layers of polyethylene, shade balls, and large canopies. Implementing these structures is not practical in large dimensions and costs much money, and is only suitable for small scales. Biological coatings, such as lilies, small duckweed, great duckweed, and watermeal, have a potential to reduce evaporation from the water surface. These coatings have not received much attention due to their very low efficiency and their effect on water quality (Youssef and Khodzinskaya 2019). The blue lentil plant can be used as an environmentally friendly cover to reduce evaporation from the water surface. The results obtained from this aquatic plant show that the vegetation of blue lentils has reduced the evaporation until about 9%. In addition, aeration increases the growth of water lentils and further reduces evaporation (Soltani et al. 2020). Fixed hexagonal coating is widely used in South Africa. The light color of these coatings reduces heat absorption. This coating is fire resistant and has a lifespan of 15 years, and its lifespan will increase if it is protected and maintained (Watts 2005). Constructing the plant windbreaks decreases evaporation from the water surface by reducing the wind speed (Elshafei et al. 2021). Shade physical coatings evaluated by the Australian National Centre for Engineering in Agriculture can be used in reservoirs smaller than 10 ha or reservoirs behind small dams, but in large reservoirs, it is uneconomical because of high costs (Craig 2005). Investigating the effect of counterweighted spheres on the evaporation rate from agricultural reservoirs in arid regions showed that in the presence of these spheres in the irrigation season evaporation from reservoirs reduced by more than 70% (Han et al. 2020). Monolayer is a thin layer with a molecular thickness of 2 nm that prevents evaporation from the water surface. Most monolayers have been described as layers with thickness of microns that retain heat and retard evaporation from water surface bodies (Fellows et al. 2015). Monolayers reduce evaporation by creating a thick layer coating on the water surface (Panjabi et al. 2016). Hexadecanol forms a layer as thick as a molecule on the surface of the water because the molecules of the monolayers have a hydrophilic head from one pole and a hydrophobic tail from the other pole. Therefore, when these molecules are placed in the water, their direction changes so that one pole is in the water and the other is out of the water (Reddy 2005). The low need for monolayers in large surfaces is one of the practical reasons for applying monolayers to reduce evaporation from the water surface; for example, 2 kg of hexadecanol can cover 1 km 2 . The use of monolayers increases the thickness of the boundary layer and the evaporation resistance. Hexadecanol and octadecanol monolayers materials are often easily and rapidly degraded by bacteria over a period of 3-4 days (Barnes 2008). Attempts to reduce evaporation from reservoirs and free water surfaces by using chemical compounds are not novel. Most research on monolayer-based evaporation mitigation was conducted in the 1950s to 1970s and focused on the application of the insoluble fatty alcohol, hexadecanol, and octadecanol (Brink et al 2017). The low cost of preparation, easy spray on the water surface, and high speed of distribution are the advantages of monolayers. The limitations of their use are their short life and the negative effect of wind on their performance. These thin films could be created by surfactants or some polymers, or a combination of both. The surfactants spread on the surface quickly and reduces the surface tension of the water. The tendency of these materials located at the boundary of the water and air creates an almost single-molecular thin layer of these materials on the surface of the water. Another idea to improve evaporation retardation is using a combination of two long-chain alcohols as monolayer composition. For instance, Ikweiri et al. (2008) on the Omar Mokhtar Dam achieved an evaporation reduction of about 17% with using a substance based on a mixture of cetyl alcohol and stearyl alcohol. Their results showed that 80 to 100 g of this substance could cover the area of 1 ha of water.
Due to technical, operational, economical, and environmental considerations, alcohol-based monolayers could be used widely as an evaporation suppression method. Although pure hexadecanol powder has been used in many field trials due to the fast-spreading rate of hexadecanol, the use of octadecanol is very limited because of its low spreading rate. To overcome this obstacle, some new thin films have been created by combination of the known alcohols with a surfactant as emulsifier. As reported in the literature, combination of the octadecanol and hexadecanol with an emulsifier can reduce evaporation from the water surface (O'Brien 2006; Barnes 2008). The surfactants facilitate spreading of the heavy alcohols on the water surface, significantly reducing the amount of the material dispersing into the bulk water instead of spreading at the interface. These materials that were located at the boundary between water and air creates a single-molecular thin layer. Due to the nanometer thickness of this layer, the amount of material used with emulsifier is very small in comparison with pure alcohols. Non-ionic surfactant polyoxyethylene (20) stearyl ether, is a common emulsifying agent, already used in the composition of monolayers (Herzig et al. 2011).
In the present study, three emulsions of heavy alcohols with Brij-35 were used as evaporation suppression monolayers. Two physical methods of the canopy and shade balls were used to evaluate the efficiency of the emulsions. The novelty of this research lies in the application of one-way and factorial analyses of variance (ANOVA) to investigate the effect of the meteorological parameters on the efficiency of evaporation suppression methods. The interaction effect of the meteorological parameters on evaporation from the water surface is also considered in the present study.

Study area
This research was conducted near Ahvaz Airport meteorological station with a latitude of 31° 32′, longitude of 48° 66′, and altitude of 16 m above sea level. In order to conduct the research and evaluate the efficiency of different methods in reducing evaporation from the water surface, class A evaporation pans with a diameter of 120.7 cm and a depth of 25 cm made up of galvanized iron were used. The experiments were performed during a 35-day period between August 27 and September 30, 2018. Due to the effect of different meteorological parameters on the rate of evaporation from the pans, it was necessary to consider all methods of reducing evaporation on the pans simultaneously. Therefore, six evaporation pans were used near the meteorological station. A wooden base of 15 cm height was placed under all of the pans, and the pans were situated on them. The location of the experiments at Ahvaz meteorological station with class A evaporation pans adjacent to the station is shown in Fig. 1.
The evaporation data can be used more confidently if the pans are located at the national standard climate stations or near the reservoir. However, the measured evaporation rate in this case is higher than the evaporation rate of the pans immersed in water (Althoff et al. 2019).
The type of pan, its location, and water quality along with temporal and spatial variations could affect the pan coefficient (Finch and Calver 2008;Althoff et al. 2019). The rate of evaporation from the pans immersed in water accurately demonstrates reservoir evaporation (Masoner and Stannard 2010). Therefore, the evaporation rate of the pans placed inside the reservoir is used to determine outside pan coefficient. Lake evaporation could be estimated using the pan coefficient technique, E lake = K.E ClassA , where E lake is evaporation from the lake (mm/day), E ClassA is evaporation from standard class A evaporation pan located on the land surface, and K is pan coefficient which is used to scale pan evaporation to real evaporation (McJannet et al. 2017). In many studies, a value of 0.7 is used for the class A pan coefficient (McJannet et al. 2017). Accordingly, the rate of evaporation from the pan can be used as a reference to compare the performance of evaporation suppression methods.
In this work, Brij-35, which melts at lower temperatures compared to Brij-78, making it easier to use, was applied to make the emulsion. On the other hand, the presence of three more -(OCH 2 CH 2 )-groups with a shorter alkyl chain in the structure of Brij-35 compared to the structure of Brij-78 makes easier its interaction with water. Table 1 presents the physical properties of Brij-35 and Brij-78.
To prepare the emulsions of alcohol/Brij-35 with 4:1 ratio of the components, a mixture of 4.0 g heavy alcohol (octadecanol or hexadecanol) and 1.0 g Brij-35 was vigorously stirred in 95.0 g of distilled water at 70 °C for about 30 min.
The suspension was then cooled to room temperature slowly. The 2:2:1 emulsion of octadecanol/hexadecanol/Brij-35 was also prepared by a mixture of 2.0 g octadecanol, 2.0 g hexadecanol, and 1.0 g Brij-35 in 95.0 g of water. These emulsions can be used for up to 6 months (Brink et al. 2017). In the following, the word Brij has been used instead of Brij-35. To prepare hexadecanol emulsion, 4% hexadecanol and for hexadecanol + octadecanol emulsion, 2% octadecanol with 2% hexadecanol were used in the above combination. Accordingly, three types of monolayers were prepared according to the mentioned instructions: hexadecanol emulsion, octadecanol emulsion, and octadecanol + hexadecanol emulsion. In order to evaluate the required concentration of mentioned compounds, the different concentrations of emulsion were prepared. The various injections performed over 2 days were compared with the control pan, which showed the positive effect of different emulsions on the evaporation reduction for concentrations above 0.2 mL. Therefore, experiments were performed for a concentration of 0.4 mL (11.4 mg/m 2 ). In this study, in addition to chemical compounds, physical methods such as canopy and shade balls were used to investigate the monolayer's efficiency. Performance of chemical and physical methods was investigated in 5 class A evaporation pans compared to the control pan. In the shade ball treatment, 100% of the pan surface was covered by shade balls with a diameter of 5-6 mm. In the canopy treatment, the distance of the pan from the canopy was  about 30 cm. Some studies indicate that using octadecanol and hexadecanol does not significantly affect water properties such as electrical conductivity, temperature, and pH (Piri et al. 2010). Therefore, in this study, only the parameters of water temperature and evaporation rate were measured.

Analysis of variance
In this study, one-way and factorial ANOVA were used to evaluate the performance of different evaporation suppression methods. At first, a one-way ANOVA was used to investigate the significant differences between evaporation suppression methods, and then a factorial ANOVA was used to investigate the effect of different meteorological parameters on the evaporation methods. All parameters must follow the normal distribution to some extent for using the variance analysis. The quantile-quantile (Q-Q) plot is a graphical technique for determining if two data sets come from populations with a common distribution. The Q-Q plot was used to investigate the normality of data by using normal distribution as the second dataset. The Q-Q plots of all the parameters used are shown in Fig. 3. Closeness of the data The independent variables must have a nominal or ordinal scale for using ANOVA. Accordingly, all meteorological parameters were divided into three levels: high, medium, and low. Various meteorological parameters, at first, were modified based on the average of each parameter. It means that the average of each parameter was subtracted from all data of that parameter. In such a case, the average data of that parameter will be equal to zero. Then, the data of each parameter was divided into three levels based on the standard deviation (SD). Values less than − 1SD were used as low level, values higher than + 1SD were applied as high level, and values between − 1SD and + 1SD were considered as intermediate values. The statistical characteristics of different meteorological parameters during the experiment period are shown in Table 2.

Efficiency of evaporation suppression methods
The results of the average evaporation from the water surface in different physical and chemical methods are presented in Fig. 4. The two physical methods of the canopy and shade balls are more efficient than the chemical methods and have been able to reduce the rate of evaporation to a considerable extent. The average evaporation from the pan in the two physical methods of canopy and shade balls were 3.84 and 4.28 mm, respectively, which compared to the control pan (9.72 mm), these two methods reduced the evaporation from the surface by 60 and 56%, respectively. Octadecanol/Brij emulsion, with an average of 6.27 mm and a 36% reduction in evaporation, performed better than the other two chemical methods. The two emulsions of hexadecanol/Brij and hexadecanol/ octadecanol/Brij also reduced evaporation by 25 and 21%, respectively. This rate of evaporation reduction is similar to the efficiency of commercial materials named water-saver (Sepaskhah 2018). Naseh and Shahidi (2019) estimated the reduction of evaporation from 50 to 70% by shadow balls, which is consistent with the results of this study. Piri et al. (2010) reported a reduction in evaporation using heavy alcohols between 40 and 55%. According to the results of these researchers, octadecanol emulsion has a higher efficiency than hexadecanol, which is similar to the results of the present study. Figure 5 shows the water temperature of evaporation pans for all physical and chemical methods. Based on Fig. 5, the water temperature of the pans containing the monolayer is almost the same as the control pan. The water temperature of the pan with canopy is less than the control pan. In the case of using shade balls, the temperature was higher on most days, which is consistent with the results of Afkhami et al. (2019). Preventing the wind from affecting on the water in the pan by covering the water surface, and absorbing more heat from the sun by shade balls and transferring it to the water, make the temperature of the shade balls treatment higher than other pans.

Comparing the evaporation suppression methods using one-way ANOVA
One-way ANOVA on different evaporation suppression methods showed that the difference between the means was statistically significant. All evaporation reduction methods are compared in pairs by Bonferroni analysis, as displayed in Table 3. According to Bonferroni test results, evaporation from the control pan significantly differed from all pans at a 99% probability level. Evaporation from the canopy pan had a significant difference with all methods used except for shade balls. Shade balls significantly differed from all chemical methods except for octadecanol, which indicated higher efficiency of this monolayer compared to the two other emulsions. According to Table 3, all chemical methods used do not significantly differ from each other. The coefficient of determination in one-way ANOVA was calculated to be 0.4, which showed that the type of evaporation suppression method can estimate about 40% of the evaporation. Therefore, the rate of evaporation from the pan, in addition to various evaporation suppression methods, is also affected by other parameters such as meteorological parameters.

Factorial ANOVA based on the main and interactional effects
Factorial ANOVA was used after converting the independent meteorological parameters from quantitative scale to qualitative mode. The results of this analysis are displayed in Table 4. According to factorial ANOVA, in addition to the significant effect of evaporation suppression methods, determined by one-way ANOVA, the parameters of temperature, relative humidity, and sunshine duration also had a significant effect on evaporation. The type of evaporation reduction method had the highest effect on the rate of evaporation based on the values of partial eta squared and F parameters. Air temperature and relative humidity had almost the same effect on evaporation. The interaction of different meteorological parameters did not have a statistically Table 5 Bonferroni test results in the mean intergroup differences of meteorological parameters *The mean difference is significant at the 0.05 level significant effect on the rate of evaporation. Based on the factorial ANOVA, the coefficient of determination was calculated to be 0.8, which showed that the proposed model can estimate about 80% of the evaporation rate according to the type of evaporation suppression method and meteorological parameters.

Mean intergroup differences of meteorological parameters
The Bonferroni test results in the mean intergroup differences of meteorological parameters are depicted in Table 5. According to the Bonferroni test, there is a significant difference between the levels of 1 and 3 and the level of 2 and 3 of temperature. There was also a significant difference between all the levels in terms of relative humidity and sunshine duration. Although the speed of wind did not have a significant effect on the rate of evaporation, a significant difference was observed between the levels of 1 and 3 and also the levels of 2 and 3. Figure 6 shows the effect of different meteorological parameters on the efficiency of physical and chemical evaporation suppression methods. The effect of different temperature levels on the rate of evaporation showed that at a low-temperature level, octadecanol monolayer had a lower efficiency than the two physical methods, but after increasing the temperature, its performance increased compared to physical methods. The other two chemical methods had weak performance at a low-temperature level so that no effect of the combination of octadecanol and hexadecanol on evaporation was observed at a low-temperature level. Based on Fig. 6b, at a low relative humidity, the octadecanol emulsion has a lower performance than physical methods, but at the highest level of relative humidity, its performance improved compared to physical methods. Among the two physical methods, similar performance was observed at a low relative humidity, but at a high level of humidity, the canopy performed better. Figure 6c shows that with increasing the sunshine duration, the performance of the two physical methods is becoming close to each other, Fig. 6 The effect of different levels of the meteorological parameters on the performance of evaporation suppression methods and the efficiency of the octadecanol is reduced. Comparing the evaporation rate with sunshine duration showed that the rate of evaporation increases from control treatment and chemical methods similarly. Figure 6d shows the effect of the wind speed on the performance of chemical methods. The octadecanol emulsion, at low wind speeds, performed similarly to the physical methods, but as the wind speed increased, its performance was severely affected, and its efficiency decreased. The results obtained in this section were consistent with the observations of Piri et al. (2010). Gallego-Elvira et al. (2013) also showed that wind of 3 m/s causes the monolayer to disintegrate and reduce its efficiency. For the two physical methods, due to the complete coverage of the water surface by the canopy and shade balls, the evaporation rate changes very small with increasing the wind speed.

The effect of wind speed on evaporation rate
According to previous researches, one of the effective parameters on the performance of monolayers was wind speed (Piri et al. 2010;Gallego-Elvira et al. 2013). In this study, the interaction of this parameter and other meteorological parameters pertaining to the performance of the octadecanol monolayer was investigated. In the factorial ANOVA, at first, only the main effects of meteorological parameters were considered on the evaporation rate from the pan containing octadecanol monolayer. In the second step, the interactions along with the main effects were added in the analysis of variance. The result of the main effects and interactions effects of meteorological parameters are displayed in Figs. 7 and 8, respectively. In Fig. 7, considering only the main effects, the wind speed has a direct effect on the rate of evaporation, so that in all cases, as the wind speed increases, the rate of evaporation also increases. Figure 8, which contains the main effects and the interactional effects of independent parameters, clearly shows the impact of the interactional effects on the rate of evaporation. In Fig. 8a, at high temperatures, the rate of evaporation increases sharply with increasing wind speed. At high temperatures (level 3), the rate of evaporation increased by more than 34% by changing the wind speed from level 2 to level 3, and with increasing the wind speed from level 1 to level 3, the evaporation rate will increase by 50%. According to Fig. 8b, at a low relative humidity, there is a significant difference between evaporation at various wind speeds. However, with increasing relative humidity, this difference decreases so that at a high relative humidity, the wind speed has a slight effect Fig. 7 The effect of wind speed on evaporation rate without considering interactional effects on the rate of evaporation in the presence of the monolayer. Figure 8c shows that in a high sunshine duration, the effect of wind speed on evaporation rate increases.

Conclusion
One of the disadvantages of using pure alcohols as monolayers is their poor spreading on the water surface. In the case of octadecanol, a high amount of powder particles needs to disperse into the bulk water, which contaminates water resources. Using the surfactant emulsifiers such as Brij-35 can significantly improve the spreading rate of the materials with much higher evaporation resistance. Based on experiments and statistical analysis, physical methods show better performance than chemical methods. Comparing the average evaporation in different methods showed that two physical methods of the canopy and shade balls had the best efficiency with a reduction of 60 and 56% in evaporation, respectively. Among the chemical methods, the octadecanol/Brij emulsion with a 36% reduction in evaporation had a better performance than two other emulsions and, in some cases, had an efficiency close to the physical methods. Although the performance of monolayers is less than physical methods, due to the advantages such as ease of use and less significant damage to the environment, monolayers can be used on a large scale. Meteorological parameters had a very high effect on the rate of evaporation from the water surface; therefore, the effect of these parameters on the performance of monolayers should be investigated. The results of factorial ANOVA showed that the effect size of evaporation reduction methods was 0.56, while temperature and relative humidity had an effect of 0.16 and 0.14 on evaporation, respectively. Investigating the effect of wind speed on the performance of octadecanol at different levels of relative humidity showed that at a high humidity, wind speed had a negligible effect on the monolayer efficiency. In contrast to relative humidity, at high temperatures, wind speed had a significant impact on the rate of evaporation from the water surface in the presence of a monolayer. In order to use the results of this study, it should be considered that the effects of wind speed and relative humidity in large reservoirs are generally higher than over the land surface. According to Fig. 6b, high relative humidity can increase the efficiency in the case of the octadecanol monolayer. However, according to Fig. 6d, increasing the wind speed has reduced the monolayer performance. In order to better analyze real conditions, the interaction effects of wind speed and relative humidity parameters should be used. Fig. 8 The effect of wind speed on evaporation rate by considering interaction effects According to Fig. 8a, which shows the interaction of relative humidity and wind speed, the efficiency of the octadecanol monolayer has improved with increasing of both relative humidity and wind speed.
Author contribution MK: manufacturing chemical solutions and performing the experiments, writing early version of the manuscript. JZ: interpreting the results, performing ANOVA, writing the final version of the manuscript. VN: designing and manufacturing chemical solutions.
Funding This work was supported by Agricultural Sciences and Natural Resources University of Khuzestan.
Data availability All data generated or analyzed during this study are included in this manuscript.

Declarations
Consent to publish The authors give their full consent for the publication of this manuscript.

Conflict of interest
The authors declare no competing interests.